From the standpoints of the necessity of reducing fossil-fuel consumption, prevention of global warming, etc., USC (ultra-supercritical pressure) plants are expected to be operated at an even higher efficiency. In particular, there recently is a strong trend toward high-efficiency coal-fired thermal power stations as 21st-century power plants. Turbine rotors, boiler members, and the like which are usable in next-generation electric-power generation with ultra-supercritical-pressure steam having a main-steam temperature exceeding 700° C. are being developed.
The related-art ferritic heat-resistant steels are no longer usable, from the standpoint of heat-resistance temperature, as heat resistance materials to be used as materials for turbine rotors exposed to steam having a high temperature exceeding 700° C. There is no way other than applying a Ni-based superalloy thereto.
Many of Ni-based superalloys are precipitation strengthening type alloys. In producing this type of alloy, a small amount of Ti or Al is added or a small amount of Nb is further added, and a precipitated phase constituted of Ni3(Al, Ti), which is called a gamma prime phase (hereinafter expressed by γ′), and/or Ni3(Al, Ti)Nb, which is called a gamma double-prime phase (expressed by γ″), is finely and coherently formed in the austenite (hereinafter expressed by γ) matrix to strengthen the system in order to obtain satisfactory high-temperature strength. Inconel (trademark; the same applies hereinafter) 706 and Inconel 718 belong to this type.
There also are alloys of the type in which the system is strengthened in a multiple manner by solid-solution strengthening and dispersion strengthening with M23C6 carbides besides precipitation strengthening with a γ′ phase, such as Waspaloy, and so-called solid-solution strengthening type alloys which contain almost no precipitation-strengthening element and in which the system is strengthened by solid-solution strengthening with Mo and W. The latter type is represented by Inconel 230.
Recently, from the standpoint of the problem concerning a difference in thermal expansion between such an austenitic heat resistance alloy and ferritic steel members or the problem concerning thermal fatigue strength, precipitation strengthening type Ni-based alloys which have a low coefficient of thermal expansion equal to or better than that of ferritic heat-resistant steels and which, despite this, are superior in high-temperature material properties to the ferritic heat-resistant steels have also been proposed as disclosed in Patent Literature 1, Patent Literature 2, Patent Literature 3 and Patent Literature 4.
Patent Literature 1: JP-A-2005-314728
Patent Literature 2: JP-A-2003-13161
Patent Literature 3: JP-A-9-157779
Patent Literature 4: JP-A-2006-124776